The difference in HDL-lipid constituents was examined in sera from the hepatic vein and femoral artery. In HDL, HDL2 and HDL3 the most pronounced difference existed in the triglyceride concentration which was significantly higher in the femoral artery as compared to the hepatic vein. The cholesterol and phospholipid concentrations of HDL2 alone but not of HDL3 were also significantly higher in the femoral artery than hepatic vein. The concentrations of apo A-I and A-II, on the other hand, remained unaltered in the blood circulation. These results, in combination with the finding of a significant decrease in triglycerides of VLDL and LDL fraction (d. = 1.006-1.063) in the femoral artery as compared to the hepatic vein suggest that the accumulation of triglycerides in HDL is due to their transfer from other triglyceride-rich lipoprotein and the accumulation of HDL2b occurs in the blood stream from the hepatic vein to the femoral artery.